A device such as a copier, a facsimile machine, or the like has an image reading unit. The image reading unit includes an optical module. The optical module includes for example, a light source, a photoelectric conversion element (for example, a CCD), a plurality of mirrors, and a lens. The light source irradiates an original document placed on a document reading table. The light emitted by the light source is reflected by the original document and the reflected light is reflected by a mirror to lead it to the CCD. The lens is disposed between the mirror and the CCD, concentrates the light reflected by the mirror, and forms an image on the CCD. A wire is connected to the both ends of the optical module. A drum around which the wire is wound is driven by rotation of a motor. Whereby, the wire is spooled, the optical module is pulled in a sub-scanning direction (the sub-scanning direction is perpendicular to a main scanning direction), and the optical module is moved parallel to the original document in the sub-scanning direction. By this, the image is read. When a cover body of an ADF unit (automatic document feeder) is closed after the original document is placed on the document reading table, the image reading unit detects (determines) the size of the original document in both the main scanning direction and the sub-scanning direction (the width and the length of the original document are detected). Namely, the size such as A4 size, A3 size, B4 size, or the like of the original document placed on the document reading table is determined.
The size of the original document in the main scanning direction (the length of the original document) is detected/determined based on a change in an output waveform of the CCD that appears at a certain position when the optical module reads the original document in the main scanning direction. The output waveform of the CCD greatly changes at a boundary between an area in which the original document exists and an area in which it does not exist when the original document is read in the main scanning direction. Namely, because the output waveform of the CCD greatly changes at the boundary, it can be known that the position at which the output waveform of the CCD greatly changes is the position of the edge of the original document.
A size sensor disposed to a rail in the sub-scanning direction on which the optical module moves is used for the detection of the size of the original document in the sub-scanning direction. A photo sensor composed of a pair of units (a light emission unit and a light reception unit) is used for the size sensor. The original document is detected by the size sensor as follows. The light emission unit of the size sensor emits the light, the original document is irradiated with the light, the light is reflected by the original document, and when the light reception unit of the size sensor receives the reflected light, it is determined that the original document exists. When the reflected light from the original document is not received by the light reception unit of the size sensor, it is determined that the original document does not exist. The size sensor is composed of a plurality of photo sensors (for example, two photo sensors). The size of the original document in the sub-scanning direction is determined based on the combination of the signals from two photo sensors.
The basic technology method for detecting (determining) the size of the original document placed on the document reading table has been explained above. When the sizes of the original document in both the main scanning direction and the sub-scanning direction are detected, a specific opening angle (closing angle) of the cover body of the ADF unit is used as a trigger. For this reason, a cover body angle detection mechanism is provided in a device in order to achieve the detection of the size of the original document when the closing angle of the cover body reaches the set angle (the specific closing angle).
In the document size detection device disclosed in Japanese Patent Application Laid-Open No. 2004-126132 (hereinafter, patent document 1), in order to improve accuracy of detection of the size of the original document in the main scanning direction in which countermeasures against external light are provided for the detection of the size of the original document, the detection of the size of the original document in the main scanning direction is achieved when the closing angle of the cover body reaches a certain angle when the cover body is closed. The light source of the optical module is turned off just before achieving the detection of the size of the original document in the main scanning direction, the output of the CCD is read while keeping this state, an incident position of the external light is detected, and this is removed. As a result, the influence of the external light is removed.
In the document size detecting device disclosed in Japanese Patent Application Laid-Open No. 2004-258386 (hereinafter, patent document 2), a scanner controller detects two opening angles, that are a first stage opening angle and a second stage opening angle, of the original document pressing plate based on a state of the optical sensor and when the detected angle is within a range of the first stage opening angle, it turns on a power supply of the light source, the CCD, and an original document size detection sensor as an original document size detection preparation process. Next, when the opening angle of the original document pressing plate is within a range of the second stage opening angle, the size of the original document in the sub-scanning direction is determined, it is determined whether or not the reflected light from the original document exists based on a threshold value set in advance, and when it is determined that the reflected light exists at a point, it is determined that the original document exists at the point. As a result, because the size of the original document can be detected at a low opening angle of the original document pressing plate and the most suitable angle in accordance with a closing operation of the original document pressing plate performed by a user, the original document size detection can be correctly performed while reducing an erroneous detection due to the external light.
In Japanese Patent Application Laid-Open No. 2007-139607 (hereinafter, patent document 3), a sensor mounting structure characterized in that a pair of transmission and reception vibrators are held in a housing hole or by a housing holder in a point-contact manner (or in a very light contact manner similar to the point-contact manner) is disclosed. By this structure, the vibration of the transmission/reception vibrators is less likely to be transmitted to the housing hole or the housing holder and whereby, a surface acoustic wave generated by one of the transmission/reception vibrators is less likely to be transmitted to the other transmission/reception vibrator (it is less likely to act as disturbance).
However, in the device disclosed in patent document 1, the CCD output is read twice, one is performed in a state in which the light source is not turned on and the other is performed in a state in which the light source is turned on, in a short time until the lifted cover body is closed. For this reason, when the cover body is quickly closed by an operator, the original document detection cannot follow the lid closing speed and whereby, a false detection occurs. Usually, it takes several milliseconds to several tens of milliseconds until the CCD or the light source for the optical module stably operates after the power is turned on. Moreover, because the number of times of reading the CCD output increases, a software process becomes complicated. In patent document 2 or 3, means to solve the above-mentioned problem are not disclosed.